Weird comet in asteroid belt targeted by Webb
Comets are rare in the asteroid belt between the planets Mars and Jupiter. But some comets do reside there, including Comet Read (238P/Read). Asteroids tend to be rocky or metallic. Unlike them, Comet Read – an icy body – sometimes looks fuzzy, displaying, like other comets, a coma and a tail. On May 15, 2023, NASA said the Webb space telescope has taken a close look at Comet Read. Webb confirmed water vapor, a first for a comet in the asteroid belt. More unexpected was what Webb didn’t find. It found no sign of carbon dioxide, a gas expected and observed in most comets.
A team of scientists in the U.S. published the peer-reviewed results on May 15, 2023, in Nature.
Weird comet 238P/Read
Comets are typically rich in water vapor and water ice, as well as gas and dust. Scientists think they may even have brought much of Earth’s original water to our planet billions of years ago. But when it comes to the rarer comets in the main asteroid belt, proof of water vapor or ice has been hard to come by … until now.
For the first time, water vapor has been definitively detected on a comet – Comet Read – in the main asteroid belt. Lead author Michael Kelley of the University of Maryland said:
In the past, we’ve seen objects in the main belt with all the characteristics of comets, but only with this precise spectral data from Webb can we say yes, it’s definitely water ice that is creating that effect. With Webb’s observations of Comet Read, we can now demonstrate that water ice from the early solar system can be preserved in the asteroid belt.
Missing carbon dioxide
There was, however, another result that surprised scientists. Webb found no carbon dioxide (CO2) on the comet. The paper stated:
Here, we present JWST [James Webb Space Telescope] observations which clearly show that main-belt comet 238P/Read has a coma of water vapor, but lacks a significant CO2 gas coma.
Carbon dioxide tends to compose about 10% of the volatile material in comets. So why is it missing in Comet Read?
The researchers pose two possible scenarios to explain the missing carbon dioxide. The first possibility is that the comet did have it a long time ago but ended up losing it because of the warmer temperatures in the asteroid belt. (Comets generally reside and originate in the Kuiper Belt and Oort Cloud, out past Neptune). As Kelley explained:
Being in the asteroid belt for a long time could do it; carbon dioxide vaporizes more easily than water ice, and could percolate out over billions of years.
The other theory is that Comet Read originated in a warmer region of the solar system where there was no carbon dioxide to begin with.
Additional observations and maybe even a mission?
The results are interesting, but do they represent asteroid-belt comets in general, or is Comet Read simply unusual? To find out, astronomers will now use Webb to look at other comets in that part of the solar system as well. This will be part of Webb’s Guaranteed Time Observations. Co-author Heidi Hammel is with the Association of Universities for Research in Astronomy (AURA). She said:
These objects in the asteroid belt are small and faint, and with Webb we can finally see what is going on with them and draw some conclusions. Do other main-belt comets also lack carbon dioxide? Either way it will be exciting to find out.
There is even the possibility of sending a robotic spacecraft mission out to one of these comets. Co-author Stefanie Milam, Webb deputy project scientist for planetary science, surmised:
Now that Webb has confirmed there is water preserved as close as the asteroid belt, it would be fascinating to follow up on this discovery with a sample-collection mission and learn what else the main belt comets can tell us.
The findings show how comets aren’t all alike as much as we might tend to think. The differences, and similarities, between comets in various parts of the solar system can provide valuable clues as to how they – and the solar system overall – first formed. And maybe even how life first originated on Earth, and perhaps elsewhere in our sun’s family of worlds.
Bottom line: The Webb telescope has observed Comet Read, a weird comet in the main asteroid belt. Among the findings … water vapor, yes. But carbon dioxide, no.
Source: Spectroscopic identification of water emission from a main-belt comet